Alternating-current induction-motor.



.B. MUOLLUM.` ALTERNATING CURRENT INDUCTION MOTOR.

APRLIOATION FILED MAR. l, w12.

Patented Jan. 7, 1913.

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A WIT/VESSES y INI/5MM@ "13. MGGOLLUM. lLTERN'A'IING CURRENT INDUCTION MOTOR.

, APPLICATION FILED MAR. 18, 1912v 1,049,506. Patented Jn. '7, 1913.

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WHYVSSES 1 ATES ALTERNATING-CURRENT INDUCTION-MOTOR.

To all whom it 'may concern:

Be it known that I, BURTON MCCOLLUM, a citizenof the United States, and a resident of Washington, District of Columbia, have invented a new anduseful Improvement in Alternating-Current Induction-Motors, of which the following is a specitication.

ln order that'induction motors shall have a high starting torque, it is necessary that the resistance of the secondary electric cir cuits shall be high, while in order that they may have a good etliciency and close speed regulation under ordinary load conditions, it is necessary that this secondary resistance `shall be low. y y L The object of the present invention is to provide a simple and reliable means whereby the resistance of the secondary circuitsn is made to change automatically from a high value under starting conditions to a relatively low value under normal working conditions. One means whereby I am enabled to accomplish this result is fully described in the following specification.

rIlhe present specification consists in part of a continuation of the disclosure of a previous application, Serial No. (361,21) filed Dec. 6, 1911, in which it was shown how,

by inserting permanently in certain conductors of the secondary circuit, resistance elements having a high positive temperature coefiicient of resistance, and characterized by an abrupt rise of resistance .at a crit-ical value of current, and by further so proportioning the resistance elements that the normal starting current is above the critical current value, and the normal full load current below said critical value, the resistance, can be made to vary automatically so as to cause the. `motor to `rive a high starting torque and high running etiicieney together with an improved power factor. In the above mentioned specification. it was Jfurther shownthat iron is a suitable material from which to make these resistance elcments, and that iron coated with nickel will give even better results. covered that cobalt, when. used in a nearly pure state, is a. good. material from which to make these resistance elements, this material possessing an advantage over iron for this purpose in that it maintains its high temperature coefficient ot' resistance to a higher temperature than does iron, and also inthe fact that its specific heat is less than that of iron, and consequently it can be Specification of Letters Patent.

Application filed March 18, 191:2A

made to heat more quickly and thus give a,

l have also dis-- Patented Jan. 7, 1913. serial No. 684,420.

more rapid rise of resistance.

My invention is fully described below, reference being made to the accompanying A6o drawings. 7 Of the drawings z-Figure l is a curve 'i showing the variation of resistance of a specimen of cobalt as a. function of the cur-l l rent flowing through it. Fig. 2 is a dia-65 grammatic representationshowing how the .an. cobalt resistances maybe connected to the" windings of a phase wound rotor. Fig. 3 shows one method ofintroducing cobalt ref" sistances into a squirrel cage rotor.l Fig. 4170 shows in diagrammatic form another embodiment of my invention. Fig. 5 is-aside elevation showing a section through A--B'f of the structure shown in Fig. 4. Fig. 6 j shows still another embodiment of my inn '75 vention in which the resistance elements are i f placed in the short circuit-ing rings. F ig.` 7 r showsa modieation of the construction shown in Fig. 6. Fig. 8 is aside elevation of al portion of the secondary, showing an; 30 alterna-te arrangements of the resistance'elej ments in the conductor bars on each end ,o the rotor. Fig. 9 is an end elevation of a j portion of the structure shown in Fig. 8. Referring particularly to Fig. 1, it will -85 be seen ho'w the-resistance of cobalt va'ries with the current passing through it. The ordinates ot the curve l represent the re .f sistance, and the abscissae the current. Itv will be observed that for small values of 90 current the resistance is not materially afJf: o fected, but as the current is increased to a, point where it begins to heat the cobalt considerably, the resistance rises abruptly, and with a comparativply small `further increase in current, the resistance rises to a manifold, value. This increase in resistance alwa'y takes place at incertain well dened critical ,fz value of current dependingk chiey onthe length and cross section ofthe resistance element. It will be readily understood, 'there-f mi fore, that if we have resistance elements 'of'l' cobalt connecte/l in series with the secondary"- winding of an induction motor, and further, it' the normal full load current of the rotor "10.5 is somewhat below that required to produce-` the abrupt rise of resistance, the total resistance of the rotor can be made relatively; low, thus giving good etlieiency and speed f A regulation under normal load conditionsall At starting, however,` the current is muchw-kl al,

larger, and the resistan'ceelcments can reads] ily bc so designed that the starting current lis above that required to roducevthe abrupt rise of resistance, in whic case thestartng resistance will be relatively high, thus giving rise to increased startingl torque, and at the same time reducing the currents during the starting period. As the motor speeds up, however, the current` decreases and finally falls to a value 'below `the above mentomatically returns substantially to its o riginal low value. It is importanty also that the amount of metal in the resistor shall be as small as practicable so that` their capacity 'that they may heat quickly and thus enable' the .motor to develop high starting torque immediatel after the current is switched on. In or er' to accomplish this and'at the same time keep the, resistance low enough to -give'satisfactory etliciency, it is usually necessaryV to make the resistance elements of very short.- length' and very small cross sec-A tion as compared to the other conductors in circuitwth which they are connected.

I F-ig. 2 shows diagrammatically how the cof balt resistors 2 may be connected to the secondary windings #3 of a phase Wound rotor when a three phase connection on the secondary side is desired. The corresponding method of connection for any number of. phases and for either ring or star connection Will .readily be understood by anyone acquainted -with 'polyphase apparatus.

.In applying this invent-ion in the case of the squirrel cage rotors, it is necessary to use av considerable number of resistors, in order that no e'lement of current may flow in" a complete circuit Without passing ,through at 40 least one of the resistors.,Y One method is to place a resistor in each bar near its end as shown in Fig. 3. Here a' resistance element 2 of cobalt having very short length and very small cross section compared to the bar 4 is inserted in the bar,-preferably'close to y its junction with the short circuiting ring 5. In the case illustrated the cobaltlresistor is connected to the copper bar 4 and to the lug 4 b soldering, andthe lug 4a is connected e ring 5 by means of the screw 6. It is evident however', that any other means of making these connections such as riveting, clamping, welding, or other well -known meanswill suiiice equally Well.`

Another embodiment of my' invention is shown diagran'ima'tically in Flg. 4,'. in which theshort circuiting ring 5 is made smaller than the distance between conductor bars on opposite sides of the rotor, aid the cobalt resistors 2 extend from the bars 4' radially inward tothe ring 5 substantially as shown.l Fig. 5 shows a section through the line A-B of the structure shown in Fig. 4. 'In the construction shown, the cobalt .resistance-2 is 55 soldered to the ring 5 and the bar 4, butany tioned. critical value, andthe resistance aufor absorbing heat shall beslmally in order -Fig. 9 is an ,ciable deterioration oi the cobalt resistors, lit-may often bey desirable to work the rethe cobalt with a'mei'al more resistant to other means designed to give Va good and permanent electrical and mechanicalv connection` Will suilice. In Fig. 6, the cobalt resist'ance elements are inserted in the short circuiting ring 5 by using rings of cobalt and providing portions of reduced cross section 2 between adjacent points of connection between ringand bars. A modiiied form of thisl structure is shown in Fig. .7 in which a continuous ring vof cobalt in the. form of a thin edgewise band 7 is used, and on the sides' of this are fastened in good electrical Contact, as by soldering for example, blocks 8 ot' copper' or other good conducting material. 'lhis has-the advantage over the structure shown in Fig. 6 in that the resistance vof the enlarged portions of thering ,is greatly reduced, tlius confining practically all of Ythe resistance of the short circuiting ring to t-he small portion 2', which can be made as lshort as desired so that the total resistance can be keptlow even While the cross section is made very small, .thus making the reduced ortions ada ted to be heated rapidly, and a so to give-a arge ratio of increase in resistance. 'i

In Fig.- 8 .is shown inside elevation a portion ofthe rotor including a'portion of the iron core 9, showing more in detaill a practical embodiment of the structure shown in Fig'. 3. Here it willbe seen that the I cobalt resistors 2 are placed in alternate bars on each end ofthe rotor.- In this ar,- rangement, the end rings are each con# nected directly .to'hal of the conductor. bars 100 without the interposition of the resistors sol that theselatter are not subjected to as great l mechanical strain as would occur if all the resistors were placed on one end of the rotor. f Inmostcases it will be desirable also to further support the short circuiting rings 5 by fixing them rigidly'to the rotor core orl spider, in which case the resistors could be] grouped in any desired manner on eitl ef f -V end of vthe rotor. The structure shown in end elevation of` a portion of that shown 1n Fig. 8. Here thc end ring 'eis reino\f'ed,showing in section one end of a copper conductor 4 anda section through one of the cobalt resistors 2.

While cobalt can be used alone as described above and its resistance made to rise to a manifold value during the starting, period while without causing any appresi'stors at a still higher temperature at start-` ing'in order to get a greater rise of resistance and a correspondingly low'starting current. In such cases, it is necessary. toA protect the cobalt from oxidation, as by completely inclesii-ig it so as to shield it from the air. Ono method of doing this, which lliave found satisfactory, is to'coat Lacasse oxidation at high temperatures; is particularly well adapted to this purpose, since -it stands comparatively high temperatures Without injury, and it also has a very high temperature coefficient .of resistance, although not as high as that of cobalt. Re sistance elements consisting of cobalt coated Kwith nickel are therefore, capable of giving 'a much greater rise of resistance than the TO- cobalt alone,'afnd this combination has been found satisfactory vfor most purposes. In some extreme ca ses Where it is necessary to carry the temperature point beyond that at Which nickel begins to deterioraterapidly, it is necessary to coat -the cobalt with something that will not'oxidize appreciably or' forni an alloy with cobalt at the highest temperatures reached. Platinum isjwell suited in such cases, and since the resistors vcan be made very small and since only a very thin coating isv required, the cost of suchv coating' is not a serious matter. y n

The number of resistors required. is,l in .25 general, equal tothevnumberof :phasesV in the rotor circuit. .In a plain 'squirr'elcage rotor, therefore, there Will be' required as many resistors as there'are bars; the squirrel cage. Special variations' of' the squirrel cage -t pefmay be used howeverjto reduce the num er of resistors. One method is touse a pluralitjgr ofrings on 'one or both ends ci the rotor' and connect each of the wh case each .pair of bars'con- Se cf one bali of the bars need be resistors;

.means of inserting the erswill readily seen by any anid'l'and in the annexed claims .I'hirh tqwcovefrbroadly all cases Where re sistancefelcments of cobalt are used .in the `'seconolaryf circuits of induction motors for thepurpcsehereinabove, set forth.

. 'Izc'lain'n i j l'. In-anxinducion motor, a secondary l'electrical circa-itlfcontaining resistances of cobalt, saidresistances being` of short length 5.0 and smallfcrcss section as' compared to the conductorsfcrmingthe rest ofthe electrical u circuit.

2. In an. lnducti'cnjnictor', a secondary Nicll.electrical-circuit containing resistanees o l of the resistors to a.

ringe to 'bars spaced. about one nele pitch the multiplies@ rotor,`

cobalt, said rcsistances being -cbbalb vsaid resistances being so .55- that',l nov element of current eanmiiow, through a complete .circuit in said',Seconde ary circuitwithout passing throughf'at least one of said resistors. y 3.111- 'an induction motor,. a secondary 60- electrical circuit containingresistances Aof cobalt havingsuch a degree of purity 'as'to be characterized by an -abruptrise yoff-fre-V sistanceat a critical value 'of current.v

4. In an induction mot-or', Aa .secondary"fihi, electrical circuit containing. rsistances ,of by an abrupt rise of A' coball characterizbd resistance at a cr'iical value of culjrentsaidr resistances beings@ proportioned tlllit the. normal starting duii'tentslin the resisto above said criticlf'v'alue andthe norme "full load currents belo-'wfsaid critical value@v 5. In an induction motor, a secondary electrical circuitly taining cobalt resi'stences, said resistance; ,being of such short l5l length and reduced cifsssection as' to heat' rapidly and-give ilsxfto a manifold increase in resistance'. within a few seconds after the current is switched on.

6. In an induction motor, a secondary electrical circuit containing cobalt resistences, the number of said resist-ences being equal to the number of phases in the second# ary electrical circuit.

In an induction motor, a secondary. 85' electrical circuit of the squirrel cage type, l said electrical circuit containingresistancesof cobalt, the number of said resistances being equal to the number of bars of the squirrel-cage. f v 9.0

'dln an induction motor, a secondary4 eectrical circuit containing resistances ofcoated by va substance which is not chemically changed,- when' heatedciu air tov the tempera ur`ej95 reached by the resistances when carryin the normal starting currents in the secondary -57 circuits offthe motor.

9. In an induction motor, a secondary electrical Vcircuit contaming res1stances .of 177,00

cobalt coated Withnickel.V

i Witnesses MINNIE LESTER,

. C. MCCOLLUM.

BURTON MCCOLLUM. .l 

